This invention relates generally to semiconductor diode lasers, and more particularly, to semiconductor diode lasers capable of operating at relatively high power levels. The basic structure of a p-n junction laser includes an active layer at the junction between a layer of p-type material on one side and a layer of n-type material on the other. A pair of parallel planes are cleaved or polished perpendicular to the plane of the junction, and the remaining sides of the diode structure are roughened to eliminate lasing in directions other than the intended one. This structure is called a Fabry-Perot cavity. If an increasing forward bias is applied to the laser diode, a current flows. Initially, at low currents, there is spontaneous emission of light in all directions. As the basis is increased, eventually a threshold current is reached at which stimulated emission occurs and a monochromatic beam of light is emitted from the junction. The beam is highly directional within the plane of the junction.
One of the most common forms of diode laser structure is called the double heterostructure. It has a semiconductor material of a relatively higher bandgap on each side of the active layer. These surrounding materials provide both an energy barrier to confine current carriers, and an optical barrier in the form of an abrupt reduction in refractive index outside the active layer.
Typically, semiconductor laser diodes are deliberately limited in their dimensions perpendicular to the axis along which light is emitted. First, the thickness of the active layer is kept small in order to keep the threshold current low. There is no known way of increasing the thickness of the active layer, which is the laser gain region, without giving up the high efficiency and low current threshold of the double heterostructure design. In the other transverse direction, in the plane of the junction but perpendicular to the direction of light output, this dimension of the gain region is kept small to force the laser to oscillate in only the lowest order transverse mode. In practice, this transverse dimension is controlled by the width of an electrode stripe on the surface of the structure, or of a narrow region of the structure doped with a material such as zinc, or by other means.
Lasers having their active regions constricted in both transverse directions are referred to as double-heterostructure stripe-geometry lasers. Since the restricted transverse dimensions define the cross-sectional area of light emission from the laser, for a given laser power the optical flux density at the facets of the structure will be increased by restricting the transverse dimensions. In addition, the narrow stripe geometry of the device results in a relatively high current density through the active region. It is well known that both the power output and the lifetime of semiconductor lasers are limited by the optical flux density and the current density through the active region. In other words, although the power output can be increased by increasing the optical flux density or increasing the current density, the lifetime of the device will be reduced as a result. Minimization of the transverse dimensions is limited by a damage threshold for the optical flux density, above which the device will be subject to serious damage. For this reason, the small light-emitting area of double heterojunction stripe-geometry lasers, which is typically about one square micron (1.times.10.sup.-6 meter), can support a laser power of only a few milliwatts. More desirable power levels of 50-100 milliwatts (mW) can be achieved only by either widening the electrode stripe or thickening the active region. However, widening the stripe results in loss of mode control, and thickening of the active region results in an increased threshold current.
Although no solution to this problem is known, other than the invention to be described, one researcher has experimented with injection lasers in which one end face is cleaved and the other is ground to a cylindrical curvature. This was reported by A. P. Bogatov et al., Soviet Journal of Quantum Electronics, Vol. 10, pp. 620-22 (1980).
It will be appreciated from the foregoing that there is a need for a semiconductor laser structure that will overcome these disadvantages. In particular, what is needed is a semiconductor diode laser structure that is capable of operation at higher power levels, but without sacrificing mode control or a desirably low threshold current. The present invention satisfies this need.